The present invention relates to a non-stop computer system having a multiple secondary memory and particularly relates to a non-stop data backup method and a system for saving maintenance data from the secondary memory in a period of online processing.
A non-stop computer system of the type which runs continuously for 24 hours for purposes such as global information processing is required. In this type system, it is necessary to perform maintenance work without stopping processing of usual work through a computer. It is therefore preferable that the maintenance work is constructed in a redundancy system.
Data backup processing is a part of the maintenance work. That is, data are saved from memory means acting as a part of a secondary memory (for example, a magnetic disk device) accumulated online to another medium (for example, a magnetic tape device), to prepare for recovery of the missing of data caused by system-down or the like. In this case, there is the necessity of such consideration that data in the memory means in a period of data backup must be updated without such lacking of consistency that data are partly new and partly old.
A method using a double disk device is used as a data backup method of the type in which usual work is not stopped. According to the method, an operating system (hereinafter called OS) of the central processing unit accesses two disks by using an online interface (in this case, a disk-access interface) at the time of processing of usual work to thereby freely read data from the two disks and freely write data into the two disks. When data are to be saved, the OS sets one of the disks to an offline state so that only one disk can be accessed through the online interface. Then, data are read from the disk set to the offline state, so that the data are saved to a magnetic tape or the like. An offline method in which data are saved from an offline disk to a magnetic tape through another processor or the like is generally used for data backup.
Examples of the data management technique in a data processing system using a computer include: a packing store file double control method disclosed in Japanese Patent Un-examined Publication No. JP-A-62-212819 filed on Mar. 14, 1986 by Nippon Electric Co., Ltd.; a double auxiliary memory device disclosed in Japanese Patent Un-examined Publication No. JP-A-2-133845 filed on Apr. 8, 1986 also by Nippon Electric Co., Ltd.; and a double file management method disclosed in Japanese Patent Un-examined Publication No. JP-A-1-207821 filed on Feb. 16, 1988 by Fujitsu Limited.
The offline data backup method is disadvantageous in cost and running performance in that the number of necessary equipments is increased so that the procedure of changing connection of one processing system to another processing system becomes troublesome and, at the same time, this method lacks both reliability and flexibility in construction of a non-stop data processing system.
It may be considered that the OS of the central processing unit which currently runs for offline work performs parallel processing of usual work and data backup work for saving data from a disk set to an offline state by using an offline interface not opened to users. In the system construction using such an interface not opened to users, however, there is much possibility that confusion may occur at the time of reconstruction of the system. When, for example, the specification for the offline interface is changed by improvement in the version of the OS without any notice to users, software maloperations will occur immediately. Furthermore, because the programming interface for accessing the offline disk is different from the programming interface for accessing the online disk, it is necessary for a program to be always aware of the fact that the offline disk is subject to be accessed. If any mistake in description is made at the time of generating of a program, there is a large risk that disk access by mistake may cause destruction of data or the like.
As described above, there is the present situation that a real system capable of performing parallel processing of data backup work and usual work in a computer system which runs continuously has not been provided yet.